Reaction injection molding (RIM) has become an important process for the manufacture of a wide variety of moldings. The RIM process is a process which involves the intimate mixing of a polyisocyanate component and an isocyanate-reactive component followed by the injection (generally under high pressure) of the mixture into a mold with subsequent rapid curing. The polyisocyanate component is generally a liquid isocyanate. The isocyanate-reactive component generally contains a high molecular weight isocyanate reactive component (generally a polyol), and usually contains a chain extender or crosslinker containing amine or hydroxyl groups. U.S. Pat. No. 4,218,543 describes one particular commercially significant RIM system, which requires the use of a specific type of aromatic amine as a crosslinker/chain extender. The preferred amine described in the '543 patent is diethyl toluene diamine (DETDA). Formulations based on DETDA are generally restricted to the lower flexural modulus range (i.e., less than about 70,000 psi at room temperature). While it is known to use DETDA in combination with other co-chain extenders in order to increase the flexural modulus of the resultant molding, the use of such co-chain extenders generally adversely affect the thermal properties of the resultant part.
In the automotive industry, the application of RIM technology has been primarily to produce vertical parts (e.g. fenders and fascias) and has typically not been used in the production of horizontal body parts (e.g., trunks, hoods and roofs). In order to be useful for the production of horizontal body parts, the molded product 1) must have high stiffness, 2) must have a high quality surface, and 3) must be able to withstand the heat generated during further processing of the part (e.g., painting and curing the paint). Typically, such a part must have a flexural modulus of 750,000 psi or higher.
Fiber glass reinforcement of polyurethane RIM parts is known. See, e.g., U.S. Pat. Nos. 4,435,349, 4,792,576, and 4,871,789. When utilizing such fiber glass reinforcement, several problems are generally encountered, not the least of which is the production of a smooth surface (see, e.g., U.S. Pat. Nos. 4,610,835, 4,644,862, 4,781,876, 4,810,444, 4,952,358, 4,957,684, and 5,009,821).
Other problems seen in using such mats in the RIM process include displacement of the mat in the mold, incomplete filling of the mold, and part distortion. It is believed that these problems are caused in part by the RIM reactants reaching a high viscosity in too short a time to completely impregnate the mat. One solution to this problem was described in U.S. Pat. No. 4,435,349. The '349 patent describes the use of a reaction mixture of a polyisocyanate, a polyol having an equivalent weight above 500, a relatively low molecular weight chain extender (such as ethylene glycol), and a delayed action catalyst. In all the examples of the '349 patent, an excess of polyol was used which had an equivalent weight in excess of 1800 and a molecular weight of no less than about 5500. The '349 patent also describes the use of a surfacing veil (note Example II) to improve surface appearance. The surface veil is used in an amount of 1 ounce per square yard (0.034 kg/m.sup.2).